It will be appreciated that air brake systems on modern railway locomotive vehicles may be generally of the type shown and disclosed in U.S. Pat. Nos. 2,958,561 and 3,272,565, which are both assigned to the assignee of the present application. Normally, the fluid pressure brake control system of a direct release type is provided for a freight locomotive including a pipe bracket mounting having a manually operable automatic brake valve device connected thereto on one side for controlling the fluid brake pressure in a train pipe and, in turn, the brakes on the locomotive and the trailing freight cars, and having a manually operable independent brake valve device connected thereto on another side for controlling the brakes on the locomotive independent of the brakes on the freight cars in a train. In an SA-26 independent brake valve device which is a small capacity, self-lapping diaphragm-operated type regulating valve, an air supply pressure is delivered to the brake cylinders at a preset controlled rate. The level of the delivery pressure is determined by the spring tension of an adjustable spring. That is, the control spring is adjusted to a specific value that is predicated in the yard or main line operating instructions established by each given railroad. In the past, manual adjustment of the delivery pressure was made by turning a slotted or socket adjusting screw mounted on the end of the control spring housing which was bolted to the valve body. If a high pressure is desired, the compression on the spring is increased so that a higher delivery pressure would be required to lap the valve. Conversely, if a lower pressure is desired, the compression on the spring is decreased so that a lower delivery pressure would be required to lap the valve. The independent brake valve control handle has two positions, namely, a release position located at one extreme end of a quadrant and a full application position located at the other extreme end of the quadrant. It will be appreciated that located between the release and full application is an application zone or sector. The more the handle is moved toward the full application position, the greater will be the application pressure. When the independent brake valve control handle is moved from the release position toward the full application position, a progressive cam effectively rotates a pivotal dog which causes the depression of a thimble member. The movement of the thimble member operates a supply and exhaust valve assembly which first seats the exhaust valve and then unseats the supply valve. Accordingly, the pressurized air from the main reservoir will flow through the unseated supply to the delivery port. The delivery port is connected to a control port of a relay valve. As air pressure develops in the delivery port, it also builds up on the inner surface of the diaphragm in the independent brake valve. The build-up of pressure on the inner surface of the diaphragm is opposed by the preset spring force exerted on the opposite side of the diaphragm. When the air pressure and the spring force become balanced, the valve assembly will move to its lap position in which the supply valve becomes seated so that further air flow from the main reservoir to the delivery port is terminated. Now if there is leakage in the line of the delivery port, the air pressure will drop so that the diaphragm assembly will move and cause the supply valve to be unseated. Thus, the main reservoir is permitted to restore the air pressure in the delivery port to the valve of the spring setting. It has been found that an engineer or trainman may deliberately circumvent the normal operation of the present SA-26 independent brake valve and may increase the brake cylinder pressure applied on the locomotive consist by overriding the preset spring force.